The present invention relates generally to aircraft gas turbine engines, and more particularly to a front fan gas turbine engine subassembly which reduces the amount of particles ingested into the engine's compressor.
A gas turbine engine includes a core engine having a high pressure compressor to compress the air flow entering the core engine, a combustor in which a mixture of fuel and the compressed ar is burned to generate a propulsive gas flow, and a high pressure turbine which is rotated by the propulsive gas flow and which is connected by a larger diameter shaft to drive the high pressure compressor. A typical front fan gas turbine engine adds a low pressure turbine (located aft of the high pressure turbine) which is connected by a smaller diameter coaxial shaft to drive the front fan (located forward of the high pressure compressor) and which may also drive a low pressure compressor (located between the front fan and the high pressure compressor). The low pressure compressor sometimes is called a booster compressor or simply a booster. A flow splitter, located between the fan and the first (usually the low pressure) compressor, separates the air which exits the fan into a core engine airflow and a coaxially surrounding bypass airflow. The bypass airflow from the fan provides most of the engine thrust for the aircraft. Some of the engine thrust comes from the core engine airflow after it flows through the low and high pressure compressors to the combustor and is accelerated past the high and low pressure turbines and out the exhaust nozzle. The flow splitter has a radially inner wall which defines the radially outer wall of the low pressure compressor air flow path casing. The low pressure compressor has a row of radially inwardly extending stator vanes attached to the flow splitter at its radially inner wall near its leading edge, followed by a row of radially outwardly extending rotor blades attached to a disc or rotor (which is connected to the smaller diameter coaxial shaft driven by the low pressure turbine), followed by additional alternating stator vane and rotor blade rows. The airfoil-shaped rotor blades compress, and necessarily turn the air flow. The airfoil-shaped stator vanes compress the air flow and straighten the airflow for the next row of rotor blades. The first row of compressor stator vanes is placed ahead of the first row of compressor rotor blades to straighten the airflow from the fan rotor blades.
An aircraft gas turbine engine will produce more thrust per pound of fuel consumed and will operate more maintenance free if only pure air enters its compressor (whether the compressor consists of only a high pressure compressor or whether it also has a low pressure compressor or even an additional medium or intermediate pressure compressor). Unfortunately the air flowing through the front fan may include rain or ice which lowers the combustor's efficiency and the air may include dust, dirt, sand or other foreign particle matter which erodes the leading edges of the compressor's rotor blades and stator vanes further lowering the engine's efficiency and requiring blade and vane replacement. In engines having a low pressure compressor forward of the high pressure compressor, it is the high pressure compressor which suffers the greater wear. Also, the problem of liquid or solid particle ingestion by the compressor typically is more acute during engine operation on or near the ground such as during takeoff.
A known particle separation technique for fan gas turbine engines is to bleed off some of the compressed airflow, particularly from the radially outer wall of the compressor casing. However, bleeding off compressed airflow degrades engine performance. Additionally, computer simulations of the trajectories of dust size particles have shown they will not always be concentrated in the radially outer wall region of the compressor airflow.
Another known particle separation technique for fan gas turbine engines is to add a row of "quarter-stage" rotor blades before the flow splitter. Such blades tend to centrifugally throw particles into the bypass airflow thereby avoiding the core engine airflow compressor rotor blades. However, such "quarter-stage" rotor blades add some additional weight and length to the engine, and the "quarter-stage"-compressed airflow which bypasses the core engine extracts an engine performance penalty (as in the case of the bleed particle separation arrangement).